In the field of radio networks, high-density arrangement of small cells to accommodate rapidly increasing mobile traffic has been investigated. Small cells have a smaller cell radius as compared to a macro cell, and operate with lower transmission power from a base station (transmission point: TP). Thus, the number of mobile terminals (User Equipments: UEs) sharing the same frequency within the cell can be reduced, and a per-terminal throughput can be improved.
However, high-density arrangement of small cells prompt greater interference power from an adjacent cell. For example, a case in which a plurality of TPs simultaneously transmits data to different UEs using the same frequency band is considered. In this case, for each UE, transmission signals from TPs other than a TP that transmits the data destined for the UE cause interference power to a desired reception signal, thereby rather decreasing the throughput.
To solve this problem, a next-generation radio communication interface such as LTE (Long Term Evolution)/LTE-A adopts CoMP (Coordinated Multi-Point transmission/reception) scheduling to suppress interference power between cells within the same frequency band (non-patent literature 1). CoMP scheduling schedules TP operation contents (transmission destination UE/transmission stop) within the same frequency band.
More specifically, a plurality of transmission point-user equipment combination patterns are evaluated using a predetermined evaluation function, and a pattern having a highest evaluation value is searched for, and is selected/output as an optimum transmission pattern.
In examples of a combination pattern shown in FIG. 14, for example, in pattern #1, UE0, UE5, UE8, . . . are assigned as transmission destinations of TP1, TP2, TP3, . . . When no transmission destination is assigned and a transmission stop is set, “Blank” is written, like TP3 of pattern #2.
In addition, the predetermined evaluation function indicates the sum of TP-specific lower evaluation values calculated based on external evaluation information that is input externally, and each lower evaluation value is a value obtained by dividing the instantaneous throughput of the transmission destination UE of the corresponding TP by an average throughput based on a proportional-fairness method (non-patent literature 2). The external evaluation information at this time represents, for example, the average throughput of each UE, the untransmitted data amount of each UE, and the TP-specific channel quality state of each UE. The channel quality state is indicated by, for example, CQI (Channel Quality Indicator) fed back from the UE (non-patent literatures 3 and 4).
To efficiently search for a combination pattern having the highest evaluation value, a method that applies a hill-climbing method to CoMP scheduling can be considered. The hill-climbing method is a search algorithm of repeating a small correction as many times as possible to obtain a desired pattern having the highest evaluation value. In this search, a small correction is a correction of the transmission destination of one of the TPs of the combination pattern to improve the evaluation value.
As shown in FIG. 15, a scheduling apparatus 50 includes a pattern generation unit 51 for generating transmission point-user equipment combination patterns, a pattern evaluation unit 52 for calculating the evaluation value of each of the generated patterns using an evaluation function, a transmission pattern selection unit 53 for holding the pattern having the highest evaluation value among the generated patterns, and an end determination unit 54 for detecting that the upper limit of an evaluation count is reached, and externally outputting, as an optimum transmission pattern, the pattern having the highest evaluation value among the generated patterns. The functions of the respective units will be described in detail below.
<Pattern Generation Unit>
The pattern generation unit 51 changes the transmission destination of only one of the TPs of the transmission pattern input from a transmission pattern selection unit (to be described later), and then outputs the changed pattern.
As shown in FIG. 16, upon receiving a start instruction from the outside of the scheduling apparatus 50, the pattern generation unit 51 performs an initialization process. In this initialization process, the selected flags of all the TPs are cleared to 0, the presence/absence of trial of each of the transmission destinations of all the TPs is cleared, and the transmission destinations of all the TPs in the internally held pattern are rewritten to indicate a transmission stop (Blank).
After the initialization process, a single TP (S_TP) in which the transmission destination is to be changed is randomly selected. Note that the TP to be selected is a TP with the selected flag “0”. Next, the transmission destination of S_TP in the internally held pattern is changed. This transmission destination is a UE in which presence/absence of trial of the transmission destination indicates “absence” in the transmission destination UE list of S_TP among TP-specific transmission destination UE lists input from the outside of the scheduling apparatus 50. Along with the change processing, the presence/absence of trial of the transmission destination is updated to the status of “presence”. When the presence/absence of trial of each of all the UEs of the transmission destination UE list of S_TP indicates “presence”, another TP is reselected as S_TP, and the same transmission destination change process is performed. The internally held pattern that has been changed is output from the pattern generation unit 51. After the pattern generation processing, the pattern generation unit 51 waits for an input from the transmission pattern selection unit 53. Upon detecting the input, the input transmission pattern is set as the internally held pattern, and the same change process is performed.
<Pattern Evaluation Unit>
The pattern evaluation unit 52 calculates, using the evaluation function, the evaluation value of the pattern input from the pattern generation unit 51. The calculated evaluation value and the evaluated pattern are output to the transmission pattern selection unit 53.
<Transmission Pattern Selection Unit>
The transmission pattern selection unit 53 selects/outputs, as a transmission pattern, the pattern having the highest evaluation value among the generated evaluated patterns.
As shown in FIG. 17, upon receiving a start instruction from the outside of the scheduling apparatus 50, the transmission pattern selection unit 53 performs an initialization process. In this initialization process, an internally held evaluation value is set to 0, and the transmission destinations of all the TPs of the internally held pattern are rewritten to indicate a transmission stop (Blank).
After the initialization process, when the evaluated pattern is input from the pattern evaluation unit 52, it is confirmed whether the evaluation value that is input together with the evaluated pattern exceeds the internally held evaluation value. When the input evaluation value exceeds the internally held evaluation value, which indicates that the input evaluated pattern is better, and thus the internally held pattern and internally held evaluation value are updated to the evaluated pattern and evaluation value; otherwise, the internally held pattern and internally held evaluation value are maintained. After the update process, the transmission pattern selection unit 53 outputs the internally held pattern and internally held evaluation value to the end determination unit 54.
<End Determination Unit>
The end determination unit 54 counts the evaluation count of the evaluated patterns. When the count value reaches the upper limit (end condition) of the evaluation count input from the outside of the scheduling apparatus 50, the end determination unit 54 determines the end of the search, sets the end flag to 1, and outputs, as an optimum transmission pattern (scheduling result), the transmission pattern output from the transmission pattern selection unit 53 at this time. Note that the end flag and the count value of the evaluation count are initialized to 0 every time a start instruction is received.
By using the scheduling apparatus 50, a transmission pattern output from the transmission pattern selection unit 53 has an evaluation value which is continuously and monotonously improved as the evaluation is repeated, as shown in FIG. 18. By this, the transmission pattern can converge to a pattern that has a high evaluation value after a sufficient numbers of evaluations are repeated.
The scheduling apparatus 50 selects a transmission pattern corresponding to the thus obtained convergence value, and outputs it as a scheduling result.
In this arrangement example, however, when selecting a transmission pattern from generated patterns, the hill-climbing method is used as a search algorithm to search for an extreme value of the evaluation values of the patterns. Therefore, even if the convergence value is not the maximum value but a relatively low extreme value, the initially found extreme value is unfavorably selected as a final convergence value, thereby ending the subsequent search. Accordingly, there is a problem that a pattern corresponding to the relatively low convergence value is selected as an optimum transmission pattern, that is, a scheduling result.